Sp. Pl. 2: 988. 1753.
Gen. Pl. ed. 5, 426. 1754.
Annuals, biennials, perennials, subshrubs, or shrubs [treelets], 1–120[–400] cm. Stems ± erect, usually branched. Leaves usually cauline, sometimes in rosettes; alternate; petiolate or sessile; blades elliptic, lanceolate, linear, lyrate, oblanceolate, obovate, ovate, rounded-deltate, spatulate, sometimes (1–)2-pinnately lobed, ultimate margins entire or toothed, faces usually hairy and gland-dotted (at least the abaxial). Heads usually radiate, sometimes ± disciform (P. alpinum), usually borne in corymbiform or paniculiform arrays (in glomerules in P. argentatum, borne singly in P. alpinum and P. ligulatum). Involucres ± hemispheric [rotate], 3–8(–12+) mm diam. Phyllaries falling, 10(–16) in 2 series, usually distinct (partially connate in P. alpinum), outer 5(–8) herbaceous to scarious, inner 5–8 ± scarious to membranous. Receptacles flat to conic; paleae cuneate to flabelliform, scarious or membranous, distally papillate and/or fimbrillate, all or the peripheral each ± enfolding a disc floret. Ray (pistillate) florets 5(–8), fertile; corollas ochroleucous, tubes stout, glandular, laminae oblong to reniform or orbiculate, or ± coroniform (none in P. alpinum). Disc florets 12–60+, functionally staminate; corollas ochroleucous, funnelform, lobes 5 (filaments distinct, anthers connate). Cypselae (black) oblanceoloid, obovoid, or pyriform, often ± obcompressed (shed together with subtending phyllary and 2 contiguous disc florets and their investing paleae); pappi 0 (shoulders of cypselae may bear 1–3 pappus-like, triangular to ovate, or ± subulate enations). x = 9.
New World, mostly warm-temperate and tropical, some temperate, introduced in Old World.
Species ca. 16 (7 in the flora).
|1||Plants 1–2 cm (plants cespitose or forming mats); heads borne singly||> 2|
|1||Plants (5–)15–120 cm (plants not cespitose or forming mats); heads in glomerules, or in simple or compound, corymbiform or paniculiform arrays||> 3|
|2||Corollas of pistillate florets bearing laminae 1–2 mm||Parthenium ligulatum|
|2||Corollas of pistillate florets lacking laminae||Parthenium alpinum|
|3||Annuals, biennials, or perennials||> 5|
|4||Leaf blades oval-elliptic to obovate (sometimes pinnately 3–7-lobed or round-toothed, ultimate margins entire)||Parthenium incanum|
|4||Leaf blades lanceolate to oblanceolate (margins mostly entire, some with 1–5 sharp teeth)||Parthenium argentatum|
|5||Perennials; leaf blades usually crenate to serrate, sometimes coarsely toothed or somewhat lobed (then mostly toward bases); pappus-like enations from shoulders of cypselae 0 or 2(–4), erect to spreading, ± subulate or threadlike||Parthenium integrifolium|
|5||Annuals or biennials (sometimes persisting); leaf blades mostly 1–2-pinnately lobed; pappus-like enations from shoulders of cypselae 2, erect, ± deltate to ovate.||> 6|
|6||Biennials (sometimes flowering first year or persisting); leaf blades ± pinnately lobed, abaxial faces strigillose (usually with erect hairs 1–2 mm as well)||Parthenium confertum|
|6||Annuals (rarely persisting); leaf blades (1–)2-pinnately lobed, abaxial faces scabrellous (seldom with erect hairs 1–2 mm)||Parthenium hysterophorus|